I would like to express my gratitude to all those who have helped me to complete this research project. The use of FRPs as externally bonded (EB) and near surface mounted (NSM) reinforcement for strengthening precast prestressed hollow plates is evaluated experimentally.
General
As no similar application was investigated to the author's knowledge, on prestressed hollow core plates, a comprehensive understanding of the behavior of EB and NSM FRP reinforced hollow core elements is needed. A comprehensive understanding of the behavior of FRP-reinforced hollow plate elements is needed so that appropriate design provisions can be developed.
Research Motivation
An experimental program was carried out in this study to contribute to the knowledge about the performance of solid core plates. Results from this study will help refine recommended code practices and explore new applications for strengthened FRP elements.
Research Objectives
Scope of Work
Research Methodology
General
Behaviour of Reinforced/Prestressed concrete in flexure and shearand shear
A crack will form in concrete when the primary tensile stress somewhere reaches the concrete's cracking strength. After diagonal cracking, the tensile stresses in the concrete are significantly reduced, where the compression field theory assumes that the primary tensile stress is zero, more superior modified compression field theory accounts for the contribution of the tensile stresses in the concrete between the cracks.
Behavior of Hollow Core Slab
History of Fiber Reinforced Polymer Composites
Behaviour of Externally bonded FRP Strengthened Con- cretecrete
The percentage increase in ultimate strength of wrapped concrete beams is a function of the number of longitudinal layers of carbon fabric. The effectiveness of externally bonded FRP for shear strengthening was found to be significantly affected by the cross-sectional shape of the beams.
Behaviour of NSM FRP Strengthened Concrete
The main failure mode for all reinforced beams was splitting of the concrete surface at the concrete epoxy interface. Splitting of the epoxy cover (splitting of the epoxy cover without cracking of concrete, cracking of the epoxy cover including fracture in the concrete along sloping surfaces, fracture of the concrete edge).
General
Test Series I
Test Series II
Specimen Properties and Characteristics
Material Properties
Laminate thickness is 1 mm (impregnated with Sikadur-330) per layer with tensile strength of 348 MPa, tensile E-Modulus of 14 GPa. The epoxy resin used to bond the GFRP sheet to the concrete was Sikadur-30 LP.
Strengthening Procedure
Spirally wound GFRP bars of 12 mm diameter were used as NSM reinforcement, with tensile strength of 700 MPa, tensile E-modulus of 49 GPa. The adhesive is a two-component epoxy resin specially designed for use at high temperatures (HDT - 820). Care was taken to align the edges of U-envelope perpendicular to the longitudinal axis of the slab.
According to the NSM layout specifications given by ACI 440.2R for NSM circular bars, the minimum groove dimensions must be 1.5 times the diameter of the bar. De Lorenzis and Nanni Lee et al found that an increase in groove size would result in an increase in average bond strength, so a conservative groove size of 24 mm was chosen. For NSM reinforcement, a concrete cutter with a diamond blade was used to make square grooves on the bottom surface of the slabs with a lateral dimension of 24 mm for different slabs at appropriate locations as shown in Figure 3.10.
After cleaning the grooves with compressed air, epoxy resin is filled halfway, then the FRP rod is placed in the groove and pressed lightly, causing the epoxy resin to flow around the rod. The groove is now filled with more resin and the surface is leveled on the concrete base.
Test Setup
Loading Procedure
Instrumentation
General
Series I
Failure progression involved: flexural cracks at the bottom in the constant moment region; Crack propagation; Formation of more widespread cracks along the length of the sample compared to the control sample; Yield of prestressed strands; Failure progression involved: flexural cracks at the bottom in the constant moment region; Crack propagation; Formation of more widespread cracks along the length of the sample compared to the control sample; Premature detachment of GFRP occurred; Specifying prestressed strands; followed by a small crushing of concrete under the loading point. There is failure progression, starting from: bending cracks at the bottom over an area of constant moment; Crack propagation; Formation of more widespread cracks along the length of the sample compared to the control sample; Specifying prestressed strands; followed by a small crushing of concrete under the loading point;
For each panel, the cracking load was determined based on the change in the slope of the load-deflection curve. The pre-crack strength was almost the same as most of the post-cracking performance of the composite for all control and strengthened specimens. The strengthening effect of the tendons provides the section with additional load-bearing capacity, which results in a positive increase in the slope (Figure 4.7).
All of the strengthened specimens showed a higher service load than the experimental crack load (Fig. 4.7). With FRP reinforcement, yielding can be delayed due to the reserve stiffness of the internal steel reinforcement due to the load-bearing contribution of the FRP. At a certain load level, existing flexural cracks became flexural-shear cracks along the shear span of the reinforced specimens.
Series II
Failure progression involved: flexural cracks at the bottom in the constant moment region; Further crack propagation, formation of distributed cracks along the length of the sample; Lower GFRP fracture; Yield of strands; Unwinding U-wraps; Development of diagonal cracks as an extension of previously existing flexural cracks (flexural-shear cracks); Sudden compression failure below the loading point occurred, as shown in Figure 6. Due to premature debonding of U-wraps, the shear strengthening was not fully utilized. Increased shear stresses at the crack tip led to the development of diagonal cracks as extensions of previously existing flexural cracks (flexural-shear cracks). The failure history was as follows: few flexural cracks in the constant moment region; Crack propagation, formation of distributed flexural cracks along the length of the specimen; flexural-shear diagonal tension cracking; Diagonal cracks developed at a distance of 2d to 4d (shear span) from the surface of the support, leading to the tearing and loosening of U-wraps; One of the diagonal cracks expanded into a main diagonal tension crack and extended to the upper compression fiber of the plate, as shown in Figure 4.12.
As the loading was further increased, debonding of the underlying GFRP occurred at a previously formed diagonal flexural-shear crack. With further loading, the load levels reached the flexural-shear capacity of the part leading to diagonal flexural-shear stress cracking. Then, more diagonal cracks developed at a distance of 2d to 4d (shear spacing) from the support face.
One of the diagonal cracks expanded into a main diagonal tension crack and extended into the upper compression threads of the slab as. Conclusions: After cracking, the internal prestressing reinforcement as well as the external GFRP lamination contributed to the increase in the load-carrying capacity; No release of the prestressed tendons was observed; Due to the premature detachment of the U-wraps, the shear strengthening was not fully utilized, the increased shear stresses at the crack tip led to the development of diagonal cracks as extensions of the pre-existing flexural cracks (flexural-shear cracking). The pre-crack stiffness for all control and reinforced specimens was almost the same since most of the composite action occurs after cracking.
General
Analytical Study
PPHC boards also fall into this class with increased complexity due to the wide range of voids. Two predominant defects that are possible in PPHC plates are due to the development of flexural shear cracks or web shear cracks depending on the ratio of shear span to depth (a/d) in the loading arrangement. Vi = factored shear force in the considered section due to externally applied load Mmax = factored moment in the considered section due to externally applied load;.
In the high shear zone (web region), the principal tensile stress directions are inclined to the longitudinal axis of the member, causing diagonal shear cracks to develop in the web when the principal tensile stress exceeds the tensile strength of concrete. These diagonal cracks initiate in the web of the PPHC plates where the web thickness is minimum, are located close to the neutral axis, and then propagate diagonally as the load increases. In series I, the predicted failure mode for plate (IS-150-7.5-EB) was the debonding of GFRP, but because the difference between the predicted debonding load and the cracking load was very small, the actual observed failure mode was the cracking of GFRP.
For slabs (I-S-150-7.5-NSM), the prediction of the failure mode (bending-shear) is not the same as the actually observed failure mode (shear failure), which may be due to the variability of the tensile strength of the concrete or of a manufacturing defect. A good correlation was observed. between the analytical and experimental load deflection response of the plates. The entire load deflection behavior was reasonably predicted, but the pre-crack stiffness was lower than the experimental one, which may be due to underestimated elastic modulus of the concrete in the analysis or limited number of sections considered for the numerical curvature integration process.
Finite Element Study
Since stress stiffness can significantly affect the results of the analysis, uniaxial stress-strain-strain curve data for high-strength concrete was generated using reliable equations proposed by Wang and Hsu [32] can be seen in Fig 5.12. The geometry of the element is defined by eight corner nodes with three translational degrees of freedom (UX, UY and UZ) at each node as shown in Fig 5.15. The prestressing strings of the PHC slabs were modeled using an element type called T3D2: a 2-node linear 3-D truss.
Abaqus searches for the geometric relationships between nodes of the nested elements and the host elements. Abaqus uses the undeformed configuration of the model to determine which slave nodes are bound to the master surface. By default, all slave nodes that are within a given distance from the master surface are bound.
The cross section of the test specimen was exactly replicated for the simulation as shown in Figure 5.17. The cross-section of the model was placed on the X-Y plane, and the Z axis was placed along the longitudinal direction of the plate. The purpose of the first loading step was to transfer the prestressing effect from the loaded strands to the surrounding concrete section, following the actual construction sequence of the PPHC slab.
This research focuses on several important strengthening concepts and failure modes of the reinforced PPHC plates. As the EB FRP reinforcement ratio increases, unfavorable debonding of laminates may occur, which can be prevented by careful design of the system in terms of flexural reinforcement limits.
